My gas furnace (Ultra SX90 Armstrong)will not work when my thermostat (Thermostat – Honeywell - RTH230B) is switched to "auto". When the thermostat is switched to "on" it runs fine (or, at least, it did until I altered the programming on the thermostat this morning-- now nothing seems to work).

Please help! I REALLLLLLY would like to avoid the hefty service charge!